K2-21 c
Astrophysical Synthesis & Habitability Characterization
K2-21 c is a confirmed sub-neptune exoplanet orbiting the star K2-21, located approximately 272.8 light-years (83.65 parsecs) from the Solar System. First announced in 2015 by K2 using the Transit detection technique (producing an estimated primary transit depth of 0.1246% across the stellar disk), this world measures 2.49 Earth radii and has a measured mass of 6.76 Earth masses. Combining its mass and radius yields a mean bulk density of 2.41 g/cm³ (an intermediate density indicative of a deep water-ice mantle or dense volatile envelope), a surface gravity of 1.09 g⊕ (10.69 m/s²), and an escape velocity of 18.4 km/s.
In its orbital architecture around K2-21, K2-21 c completes one full revolution every 15.502 Earth days at a semi-major axis of 0.107 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 75.1 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.21 carries the world between 0.0845 AU at periapsis and 0.1295 AU at apoapsis, driving measurable seasonal insolation cycles.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 420 K (+147°C / +297°F) and receiving 5.10 S⊕ (~6,941 W/m²), K2-21 c orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star K2-21 is classified as a low-mass red dwarf of spectral type M0.0 with an effective photospheric temperature of 4,222 K, a radius of 0.6468 R☉, a mass of 0.676 M☉, an estimated bolometric luminosity of 0.119 L☉. Stellar evolution modeling indicates an estimated stellar age of 1 Gyr and a metallicity of [Fe/H] = -0.11 dex (metal-poor relative to the Sun). From Earth, K2-21 shines at apparent V-band magnitude 12.851 (faint and requiring research-grade or CCD telescopic photometry) in the Southern celestial hemisphere. The K2-21 system is a multi-planet architecture hosting 2 confirmed worlds, with K2-21 c sharing its stellar neighborhood alongside K2-21 b.
Methodology: Computed using the Schulze-Makuch et al. (2011) weighted metric comparing physical interior (radius, estimated density) and surface thermal regime (escape velocity, equilibrium temperature).
Discovery Facility: K2 · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | K2-21 |
| Spectral Type | M0.0 |
| Effective Temp (Teff) | 4,222 K |
| Stellar Radius | 0.6468 R☉ |
| Stellar Mass | 0.6760 M☉ |
| Bolometric Luminosity | 0.119 L☉ |
| Metallicity [Fe/H] | -0.110 dex |
| Stellar Age | 1.000 Gyr |
| Apparent Magnitude | V: 12.851 · G: 12.279 |
| System Architecture | 1 Star, 2 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.107000 AU |
| Orbital Period | 15.502 days |
| Mean Orbital Velocity | 75.1 km/s |
| Orbital Eccentricity (e) | 0.21000 |
| Orbital Inclination (i) | 88.8500° |
| Mean Bulk Density | 2.41 g/cm³ (18.4 km/s vesc) |
| Insolation Flux | 5.1000 S⊕ (~6,941 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 272.800 LY (83.645 pc) |